The invention herein resides in the art of devices and techniques for decoding spread spectrum signals and, more particularly, to such apparatus and techniques for synchronizing decoding signals generated by a receiver with a received spread spectrum signal. Specifically, the invention relates to a correlation system adapted for use with portable battery operated commuting and communication systems which typically employ burst transmissions such as radio packet data networks and wireless LANs.
Apparatus and techniques for direct sequence, spread spectrum modulating and demodulating data signals are well known. Modulation involves generating a periodic comparatively high frequency, repetitive pseudo noise code (PN code) and effectively mixing the data signal with a PN code as with an exclusive OR gate or a balanced mixer. The resulting signal is characterized by a very wide bandwidth and very low spectral energy density. To decode or demodulate a received spread spectrum signal, it is necessary to generate a decoding signal corresponding to the particular PN code previously used for encoding purposes and to apply both the received signal and the PN code to a balanced mixer or other means commonly referred to as a correlator.
It is known that spread spectrum modulation and demodulation are particularly useful for the transmission of data signals such as analog voice signals or digitized data in building interiors over radio frequency (RF) carriers. The low spectral density characteristic of such signals reduces the tendency for interference with other radio sensitive equipment. Additionally, spread spectrum techniques are known intrinsically to reduce interference between multiple reflected versions of a transmitted signal, as minimal phase differences between the local PN decoding signal and reflected signals results in low signal correlation and consequently demodulation of such multiple signals.
Acquisition and code tracking of Direct Sequence Spread Spectrum has traditionally been performed by one of two methods: (1) serial acquisition or (2) matched filter/full parallel acquisition. Serial acquisition has traditionally been used for continuous transmission systems where long acquisition times do not impair system performance. Matched filter/full parallel acquisitions have been generally applied to burst transmissions such as those used in radio packet data networks and wireless LANs where PN code lengths and data spreading ratios are greater than 100 chips. The disadvantage of the parallel approach is the excessive hardware complexity for a digital implementation and the high insertion loss and inflexibility to change data bandwidths and chip and signaling rates for SAW (Surface Acoustic Wave) matched filter implementations.
In general, prior art structures and techniques are not conducive to implementation in portable battery operated commuting and communications systems. Wireless packet networks and LANs use short PN codes. Accordingly, it appears that a hybrid multiple serial correlator approach may be used to increase acquisition rates, and reduce hardware complexity while providing complete flexibility to perform symbol, chip rate and code length changes within a packet burst.
In the prior art, as in U.S. Pat. No. 4,774,715, a basic multiple correlator acquisition system based on analog correlators is shown, but the same fails to provide the desired characteristics of optimal size and power reduction necessary for use in a portable battery operated system.
In light of the foregoing, it is a first aspect of the invention to provide a correlation system for use in wireless direct sequence spread spectrum systems which is adapted for use in wireless packet networks and LANs.
It is another aspect of the invention to provide a correlation system for use in wireless direct sequence spread spectrum systems which allows for an increase in acquisition rates over prior structures and techniques.
A further aspect of the invention is the provision of a correlation system for use in wireless direct sequence spread spectrum systems which greatly reduces hardware complexity and the cost, maintenance, and reliability concerns incident therewith.
Yet a further aspect of the invention is the provision of a correlation system for use in wireless direct sequence spread spectrum systems which greatly reduces power consumption over prior art structures and techniques.
Yet an additional aspect of the invention is the provision of a correlation system for use in wireless direct sequence spread spectrum systems which provides the flexibility and capability to perform symbol, chip rate, and code length changes within a packet burst and which allows the symbol period and code repetition period to be equal such that acquisition of the codes provides concurrent recovery of the symbol timing.
Still another aspect of the invention is the provision of a correlation system for use in wireless direct sequence spread spectrum systems which multiplexes a plurality of low precision correlators to periodically seek alternate PN codes exhibiting a higher level of responsiveness and finely adjust the system time clock by testing small time increment delays or shifts on either side of a selected PN code.
The foregoing and other aspects of the invention which will become apparent as the detailed description proceeds are achieved by: a direct sequence correlation systems, comprising: first means for receiving an RF signal and generating analog in-phase (I) and quadrature (Q) signal components; second means receiving and digitizing said I and Q signal components; a high precision correlation circuitry receiving said digitized I and Q signal components and providing signal demodulation thereof, a low precision correlation circuit receiving said digitized I and Q signal components and providing code search and code tracking for a PN code associated with said RF signal; and a synchronization processor interposed between said high and low precision correlation circuits for synchronizing said high precision correlation circuit with said PN code.
For a complete understanding of the objects, techniques, and structure of the invention, reference should be made to the following detailed description and accompanying drawing wherein a schematic diagram of the direct sequence correlation system according to the invention is shown.